• photosynthesis makes both carbon and energy available to living organisms and produces the oxygen in the atmosphere using water (or H2S), CO2 and the light energy absorbed by the photosynthetic pigments in the living cells.

      6CO2 + 6H2O--------------> C6H12O6 + 6O2


  • Almost all life on earth depends on photosynthesis either directly or indirectly as in the case of animals.
  • Photosynthesis is an anabolic reaction. hence this requires enegy.
  • The energy and the hydrogen are supplied by the light dependent reaction which makes ATP and reduced NADPH.


To explain the process of light dependent reaction, we should know about the photosystem.

  • The photosynthetic pigment molecules(chlorophyll-a, chlorophyll-b,carotene, xanthophyl) are located in two types of photosystems on the thylakoid membranes of the chloroplast. the two photosystems are known as,

                           photosystem I (PS I) and

                           photosystem II (PS II).

  • The photosynthetic pigments collect light energy.
  • Different pigments collect light of different wave lengths, making the process more efficient.
  • All the energy that are absorbed by the photosynthetic pigments is finally transferred to a specilised form of chlorophyll-a(reaction centre)known as;

                               P700 in PS I
                               P680 in PS II
                    (P stands for pigments)


The light dependent reaction.

 light reaction of photosynthesis

  • The light dependent reaction takes place on the thylakoid membrane of the chloroplast.
  • The main process of the ligt reaction is....the chlorophylls P700 and P680 become excited by the light energy they absorb and release high energy electrons. The fate of the released electrons explains the remaining part of the process.
  • The process depends on a flow of electrons from P680 and P700. hence, the P680 is excited first and then the P700.
  • First, an electron from P680 or P700 is boosted to a higher energy level, that is it acquires excitation energy.
  • instead of falling back to the photosystem and losing its energy it is captured by an electron acceptor. This represents the important conversion of light energy to chemical energy.
  • The electron acceptor is thus reduced and a positively charged(oxidised) chlorophyll molecule is left in the photosystem.
  • the electron then travels downhill, in energy terms, from one electron acceptor to another in a series of oxidaton-reduction reactions. 


  • the energy lost during this electron flow is coupled to the formation of ATP.The process of the formation of ATP is called phosphorylation. 

     ADP+ Pi + energy------------> ATP

  • The pathway followed by the elctron can be cyclic, returnig to where it began, or non cyclic, ending at NADP. When the boosted electron from the P700 is added to NADP it is changed to NADPH.

     NADP+H++e+ energy-------------->NADPH

  • The hydrogen comes from water during a process called photolysis that occurs when the P680 chlorophyll is being oxidised.

      2H2O--------------->4H+ + 4e+ O2

  • The oxygen is released to the atmosphere and the electron reaches the oxidised P680 and reduces it.
  • And the electron which is boosted from the P680, reaches the P700 finally and reduces it.

Non cyclic photophosphorylation

 non cyclic

  • Excited electrons from P680 and P700 reduce electron acceptors X and Y respectively so that P680 and P700 become positively charged.
  • The electron donor which provides the replacement electron for P680 is water.
  • Water is split, releasing electrons which enter P680.
  • Hydrogen ions and Oxygen are also released.
  • Oxygen escapes as a waste product.
  • Electrons flow from X along a chain of electron carriers losing a little energy each time they move from one carrier to the next.
  • Eventually they fill the positive holes left in P700.
  • The energy from this flow is coupled to ATP production.
  • Electrons also pass downhill in energy terms, form Y to NADP along a chain of electron carriers and combine with hydrogen ions from water to form NADPH.



Cyclic photophosphorylation.



  • In cyclic photophosphorylation, electrons from Y are recycled back to P700 via another chain of electron carriers.
  • As the electron pass down the chain their excitation energy is coupled to ATP production just as in non cyclic photophosphorylation.
  • Extra ATP can be made via cyclic photophosphorylation.





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